The Gallatin River is a tributary of the Missouri River, approximately 120 mi (193 km long), in the U.S. states of Wyoming and Montana. It is one of three rivers, along with the Jefferson and Madison, that converge near Three Forks, Montana, to form the Missouri. The river was named in July 1805 by Meriwether Lewis at Three Forks. The eastern fork of the three, it was named for Albert Gallatin, the U.S. Treasury Secretary from 1801–14. The western fork was named for President Thomas Jefferson and the central fork for Secretary of State James Madison.